1. Field of the Invention
The present invention relates generally to communication systems, and more specifically to a communications system suitable for transmitting data between a remotely operated semi-submersible vehicle and a data gathering ship.
2. Description of the Prior Art
Marine seismic operations are typically conducted by providing a surface vessel which tows a long streamer cable through the water. A plurality of marine pressure geophones (commonly referred to as hydrophones) are mounted along the streamer at spaced intervals. The streamer is made neutral buoyant at a desired depth, typically on the order of 10-20 meters below the surface of the surrounding body of water. The length of the streamer being towed by the surface vessel can be as long as several thousand meters. An energy source can be carried by the surface vessel in order to produce aquatic waves that generate the necessary seismic reflections which are detected by the hydrophones. The data so gathered is recorded and processed to generate a seismic profile.
There are inherent technical and economic limitations to gathering marine seismic data through the use of a surface vessel to tow a single streamer cable. At the present time, multiple streamers are commonly towed behind surface seismic vessels in an effort to acquire more reflection data during a single seismic run. A technical disadvantage of the marine streamer method as presently practiced is that it cannot be employed to produced a so called "split spread" wherein the shot point is at, or perpendicularly offset from, the center of the hydrophones deployed in the water on a single path. A further limitation of prior techniques using single streamer cables is the difficulty of acquiring three dimensional seismic data. The amount and usefulness of data which can be obtained from a streamer towed behind a vessel which is also the sound source is limited.
The cross referenced application, SEMISUBMERSIBLE REMOTELY OPERATED VEHICLE AND METHOD, incorporated herein by reference, describes a design for a remotely operated semi-submersible vehicle which can be used for towed hydrophone streamers. Multiple streamers may be positioned to both sides of the main surface vessel, often referred to as a mothership, in order to obtain improved seismic profiles. The advantage of using such a system will become immediately apparent to those skilled in the art.
The data gathered by the hydrophones must be communicated to the mothership for processing and storage. Since each hydrophone streamer is attached to an independently piloted semi-submersible vehicle, no direct electrical connection can be made with the mothership. Communicating the collected data to the mothership can be a significant problem. Data is gathered by the hydrophones at a relatively high rate. Data gathering is an intermittent process so that the average data rate, while being relatively high, is significantly less than the peak data rate.
In addition to the high data rates which must be employed, a significant problem with communications between a remotely operated semi-submersible vehicle and the mothership is the noisiness of the communications channel. Microwave communications pose problems due to multipath distortion from a few fixed reflectors that cause interfering signals. In water-borne applications, the common approach is to try to avoid these reflections by preventing the beam from intersecting the waters surface. In the case of a remotely operated semi-submersible vehicle, the transmitting antenna on the vehicle is necessarily very low to the surface of the water and makes prevention of the beam intersection with the water surface nearly impossible. Furthermore, there will nearly always be a large number of multiple paths as the surface of the water appears as a scintillating surface at the low look angles of typical antennas. In addition, obstacles which block the line of sight between the mothership and the remotely operated vehicle will also interrupt the transmission of data. Such objects can include, for example, other ships and offshore drilling rigs. Shorter outages can also occur due to wave action and ship motion.
Any loss of data due to an outage renders the collected data nearly useless. Since it is generally not feasible to store all collected data in the remotely operated vehicle until it recovered by the mothership, it is necessary to devise a communication system that detects outages in the communication channel and recovers from them.
It would therefore be desirable to provide a data communications system suitable for transmitting data between a remotely operated seismic vehicle and a mothership which has the capacity to handle the high rates of data found in hydrophone streamers. It would further be desirable to provide such a communication system which can transfer data correctly over a noisy channel, and which can recover from communications channel outages.